Method of processing gas associated with oil

ABSTRACT

The invention relates to a method for producing combustible gas for gas engines from associated gas obtained during oil production, the associated gas containing methane, ethane, propane, hydrocarbons having more than three carbon atoms, and optionally propene, wherein a gaseous fraction and a liquid fraction are obtained by partially condensing the associated gas, wherein the condensation process is performed under such pressure and temperature conditions that the liquid phase is substantially free from methane, ethane, propane, and optionally propene, and that substantially the entire methane, ethane, propane, and optionally propene are contained in the gaseous phase.

The invention concerns a method of producing combustible gas for gasengines from associated gas which is produced in crude oil productionand which contains methane, ethane, propane, hydrocarbons having morethan three carbon atoms and optionally propene, wherein a gaseousfraction and a liquid fraction are obtained by partially condensing theassociated gas.

In crude oil production from crude oil deposits by means of crude oilproduction stations an associated gas is produced, which almostexclusively consists of hydrocarbons. This involves a mixture ofdifferent gaseous hydrocarbons, primarily alkanes and alkenes. Thatassociated gas is frequently referred to as “associated petroleum gas”or APG, and earlier also as “flare gas”.

It will be noted however that this associated gas is unsuitable forfeeding into a gas pipeline as, in comparison with natural gas whichcontains methane as its main component, it has a complex mixture ofdifferent hydrocarbons. Consumers which are optimized for given gascompositions and gas qualities cannot be optimally operated with suchmixtures. Therefore that associated gas has long been of lowsignificance for energy utilization. For many years it was burnt withoutbeing put to use, and the earlier term of “flare gas” is also derivedfrom that consideration. That method is not only environmentally harmfulbut it also represents a waste of valuable resources.

A possible way of using the associated gas involves combustion inboilers operated with gas burners. It will be noted however that thethermal energy requirement at the oil production stations which aresituated in the most exposed locations is generally not as great asthere is associated gas. Direct utilization of the gas, by it beingburnt in gas engines operated on the basis of the Otto cycle in order toproduce power by means of a generator as a further consequence fallsfoul of the excessively low methane number of the associated gas. Themethane number is a measurement in respect of anti-knock quality and anexcessively low methane number means that the anti-knock quality of theassociated gas is excessively low to convert it into power and heat withco-generating heat and power installations.

To make the associated gas useable for combustion in a gas engine usingthe Otto cycle a gas processing installation generally has to bedisposed upstream of the gas engine. Different technologies are knownfor that purpose, inter alia the membrane technology or cooling the gasto extremely low temperatures. In that respect it is usual to producealmost natural gas quality, that is to say to increase it to a methanecontent of over 85%. Deposited higher-grade hydrocarbons can further beused as a valuable substance by cracking or by direct thermalutilization.

The membrane technology for processing associated gases represents arelatively high technical and financial involvement. The alternative,cooling the gas with the aim of providing that the higher-gradehydrocarbons condense out, is also very complicated and expensive,nonetheless requiring very low temperatures of the order of magnitude.Thus for example propane (C₃H₈) becomes liquid at −42 degrees Celsius,and ethane (C₂H₆) even becomes liquid only at −89 degrees Celsius. Suchtemperatures can be produced for example with turbo-expanders. That alsorequires a very high level of technical and financial implementation.

WO 2007/070198 A2 presents a method of processing associated gas incrude oil production. That method primarily involves producing amethane-rich gaseous phase and a liquid phase with a low methanecontent. In that respect different pressure and temperature conditionsare referred to, wherein attention is primarily directed to very lowtemperatures and very high pressures so that this affords a gaseousphase containing almost exclusively methane and small traces of ethane.It will be noted however that the liquid phase produced in that casestill has a high methane and ethane content, which can be deduced fromthe described method conditions and examples. That however is alsounderstandable as the production of a liquid phase with a high usefulenergy value is in the forefront in WO 2007/070198 A2.

The methods applied hitherto, in which the associated gas was cooled tosuch low temperatures and pressures that almost pure gaseous methane anda condensate with the other hydrocarbons is produced, is extremelycomplicated and expensive in energy terms as cooling to the boilingpoint of ethane or ethene is required to achieve enrichment of methanein the gaseous phase. WO 2007/070198 A2 allows certain residues ofhydrocarbon compounds with two carbon atoms in the gas. It will be notedhowever that the choice of the method conditions provides that theliquid phase has a very high methane content which is naturally at theexpense of the quality of the gaseous phase obtained. The gaseous phaseobtained is therefore suitable for implementation in a gas engine inview of the methane number but the complicated and expensive methodconditions make operation of the gas engine uneconomical. The high-gradeliquid phase obtained however still has to be transported away.

The object of the present invention is to improve the method of the kindset forth in the opening part of this specification such that theproducts obtained can be better employed in further utilization. Inparticular the invention seeks to provide that the gaseous fractionobtained can be burnt in a gas engine operated on the basis of the Ottocycle in order to produce power and heat by way of a co-generating heatand power installation. In that respect production of the gaseousfraction is to be as inexpensive as possible from the economic point ofview by making use of the prevailing temperature and pressure conditionsof the associated gas obtained in oil production.

In a method of the general kind set forth in the opening part of thisspecification that object is attained in that the condensation processis performed under such pressure and temperature conditions that theliquid phase is substantially free from methane, ethane, propane andoptionally propene, and substantially the entire methane, ethane,propane and optionally propene is contained in the gaseous phase.

The basic idea of the invention, in the production of associated gas, isto transfer hydrocarbons with a high methane number into the gas phaseand to condense out hydrocarbons with a low methane number. Thatprovides a gaseous mixture which contains substantially all the methane,ethane, propane and possibly propene. In that way the methane number inthe gaseous phase and thus also the anti-knock quality of the gas can beincreased. The particular realization is that hydrocarbons with morethan three carbon atoms enormously reduce the anti-knock quality whilehydrocarbons with up to three carbon atoms can be excellently well usedin a gas engine. In particular n-butane and isobutane are responsiblefor the anti-knock rating being reduced while a gas with the mainconstituents methane, ethane and propane (optionally also propene) has amethane number which is excellently well suited for implementation ingas engines, in particular also those with upstream-connected compressordevices. The aim therefore is to separate off n-butane and isobutane.

In contrast to the state of the art therefore a particularly high-gradegaseous phase can be obtained in that way, with the valuableconstituents methane, ethane and propane, while the liquid phasecontains the higher-grade hydrocarbons, wherein that phase is stillexcellently well suited for possible further utilization. From thatpoint of view the liquid phase with a high methane content, as isobtained in accordance with WO 2007/070198 A2, is a waste whenconsidered in energy terms as gas engines are markedly more sensitivethan installations for combustion of the liquid phase produced. The gasobtained in accordance with WO 2007/070198 A2 is also not optimum interms of energy yield as a large part of the methane is lost.

It is preferably provided in accordance with the invention that thecondensation process is performed at a temperature of between −5 degreesCelsius and −14 degrees Celsius. Particularly preferably the temperaturerange is between −7 degrees Celsius and −14 degrees Celsius. As theboiling point of isobutane is −11.7 degrees Celsius, it is thereforedesirable if the temperature is below −11.7 degrees Celsius. In manygeographical areas of use of the method the prevailing ambienttemperatures are in the region of those values so that expensive coolingcan thereby either be entirely avoided or can be effected incorrespondingly inexpensive fashion, thereby permitting economicallyparticularly advantageous production of the gaseous fraction.

It is preferably provided that the condensation process is effected in aplurality of stages, wherein cooling is effected to a temperature ofbetween −5 and −8 degrees Celsius in one stage and to a temperature ofbetween −8 and −14 or −12 degrees Celsius in a further stage. In thatway low-boiling hydrocarbons can be collected in a first fraction andhigher-boiling hydrocarbons can be collected in a second liquidfraction. In that way however it is also possible for any watercontained in the associated gas to be condensed in a first step.

It is advantageous under the above-selected temperature conditions if atthe same time certain pressure conditions are set as temperature andpressure are responsible for the condensation characteristics of gases.It is preferably provided in that respect that the pressure in thecondensation process is between 1 bar and 16 bars, preferably between 10bars and 16 bars, particularly preferably between 14 bars and 16 bars.In many cases the associated gas produced in crude oil production issuesat a pressure in the region of those values so that the choice of thatrange permits economically particularly advantageous production of thegaseous fraction. In addition the costs of pressure equipment forpressures to be generated of up to 16 bars are relatively low incomparison with pressure equipment in a higher pressure equipmentcategory.

It is further preferably provided that the gaseous fraction has amethane number of at least 40, preferably at least 45. The methanenumber as a measurement of the anti-knock quality of the engine isimportant for the further purpose of use. In the present case thegaseous fraction obtained is in fact to be used in a gas engine so thatthere should be a methane number of at least 40, preferably at least 45.

In addition in a variant it can be provided that the gaseous phase issubstantially free from n-butane and isobutane.

In a further variant it can be provided that water vapor which ispossibly present in the associated gas is removed. In that respect it ispossible to use for example absorption agents and molecular sieves likezeolites or known drying agents like inorganic salts. A composition of apossible associated gas from an oil production station will now bedescribed by reference to an example in following Table 1:

TABLE 1 Composition of an associated gas (Example): Compound Mol. % CO₂0.05 N₂ 3.78 Methane CH₄ 36.67 Ethane 15.29 Propane 23.17 Isobutane 6.16n-Butane 9.73 Isopentane 2.08 n-Pentane 1.88 Hexane 0.76 Cyclohexane0.06 Heptanes 0.18 Octanes 0.14 Nonanes 0.04 Decanes and higher-grade0.01 hydrocarbons Total 100

The mixture described in the Table has a methane number of 32.7 and isnot suitable for combustion in an Otto-cycle gas engine. After coolingto about −14 degrees Celsius there was a gaseous mixture which almostexclusively consisted of methane, ethane and propane and has a methanenumber of 45. In addition the gas contains traces of carbon dioxide andnitrogen. The other constituents are almost completely contained in thecondensate. Following Tables 2 and 3 also show once again the differencebetween the subject-matter of the invention and the state of the art.

TABLE 2 Gas processing by cooling to −12 degrees Celsius CompoundBoiling point (° Celsius) H₂ Hydrogen −253 N₂ Nitrogen −196 CO Carbonmonoxide −192 O₂ Oxygen −183 CH₄ Methane −162 C₂H₄ Ethene −104 C₂H₆Ethane −89 CO₂ Carbon dioxide −79 C₃H₆ Propene −48 C₃H₈ Propane −42C₄H₁₀ Isobutane −12 C₄H₁₀ n-Butane −1 C₅H₁₂ Isopentane 28 C₅H₁₂ Pentane36 C₅H₁₂ n-Pentane 36 C₆H₁₄ Hexane 69 C₆H₆ Benzene 80 C₇H₁₆ Heptane 98

TABLE 3 Gas processing by cooling to −48 degrees Celsius CompoundBoiling point (° Celsius) H₂ Hydrogen −253 N₂ Nitrogen −196 CO Carbonmonoxide −192 O₂ Oxygen −183 CH₄ Methane −162 C₂H₄ Ethene −104 C₂H₆Ethane −89 CO₂ Carbon dioxide −79 C₃H₆ Propene −48 C₃H₈ Propane −42C₄H₁₀ Isobutane −12 C₄H₁₀ n-Butane −1 C₅H₁₂ Isopentane 28 C₅H₁₂ Pentane36 C₅H₁₂ n-Pentane 36 C₆H₁₄ Hexane 69 C₆H₆ Benzene 80 C₇H₁₆ Heptane 98

As can be seen from Tables 2 and 3 the subject-matter of the invention(Table 2) provides that all gaseous constituents with boiling pointsbelow isobutane are collected in the gaseous phase and those with aboiling point thereabove are collected in the condensate. In the stateof the art (Table 3) which involves cooling to −48 degrees Celsiusenrichment in the gaseous phase is effected exclusively in respect ofthose compounds which have a boiling point of CO₂ and below,concentration of the other constituents occurs in the condensate. Inaccordance with WO 2007/070198 A2 only enrichment in respect of CH₄ inthe gaseous phase would be observed at all, clean separation is noteffected.

The applicant's calculations showed that, with typical associated gaswhich occurs in oil production stations, only few higher-gradehydrocarbons have to be separated off to be able to operate moderninternal combustion engines which are designed or adapted specificallyfor gases with a low resistance to knocking. These include heptane(C₇H₁₆), benzene (C₆H₆), n-pentane and isopentane (C₅H₁₂) as well asn-butane and isobutane (C₄H₁₀). All those components already condense at−12 degrees Celsius so that it is sufficient for the gas to be cooleddown to that temperature. That can be implemented inexpensively and witha low level of complication with commercial refrigerating machines suchas for example water chillers and with commercially available heatexchangers and condensate locks.

The advantage of the method is that the gas does not have to be cooleddown to very low temperatures as was otherwise usual but only to betweenabout −5 and −14 degrees Celsius, preferably −12 degrees Celsius, tomake the gas suitable specifically for internal combustion engines inregard to anti-knock rating. That achieves a marked reduction in costs.

1. A method of producing combustible gas for gas engines from associatedgas which is produced in crude oil production and which containsmethane, ethane, propane, hydrocarbons having more than three carbonatoms and optionally propene, wherein a gaseous fraction and a liquidfraction are obtained by partially condensing the associated gas,characterized in that the condensation process is performed under suchpressure and temperature conditions that the liquid phase issubstantially free from methane, ethane, propane and optionally propene,and substantially the entire methane, ethane, propane and optionallypropene is contained in the gaseous phase.
 2. A method as set forth inclaim 1 characterized in that the condensation process is performed at atemperature of between −5 degrees Celsius and −14 degrees Celsius.
 3. Amethod as set forth in claim 2 characterized in that the condensationprocess is performed at a temperature of between −7 degrees Celsius and−12 degrees Celsius.
 4. A method as set forth in claim 1 characterizedin that the condensation process is effected in a plurality of stages,wherein cooling is effected to a temperature of between −5 and −8degrees Celsius in one stage and to a temperature of between −8 and −12degrees Celsius in a further stage.
 5. A method as set forth in claim 1characterized in that the pressure in the condensation process isbetween 1 bar and 16 bars, preferably between 10 bars and 16 bars,particularly preferably between 14 bars and 16 bars.
 6. A method as setforth in claim 1 characterized in that the gaseous fraction has amethane number of at least 40, preferably at least
 45. 7. A method asset forth in claim 1 characterized in that the gaseous phase issubstantially free from n-butane and isobutane.
 8. A method as set forthin claim 1 characterized in that water vapor which is possibly presentin the associated gas is removed.
 9. A method as set forth in claim 8characterized in that the water vapor is removed by condensation,absorption or combinations thereof.